Microelectronic environments generally employ micro-bonding techniques to "hand-craft" circuitry that conduct electricity to perform an intended function. Electronic products (such as computers) are constantly shifting toward higher density packaging of electronics hardware in order to achieve higher performance, while reducing the product size. This has led to high-density circuitry, micro-wiring and Very-Large-Scale-Integration all of which require line dimensions significantly below 100 microns (100 microns is of the order of the size of a human hair). Because of the small circuitry dimensions and the high performance materials used, high density circuit modules are normally very expensive production parts.
Micro-bonding is the process of joining very-fine metallic components. In microelectronics microbonding is used to modify existing circuitry. For example, microbonding can be used to repair an "open" (discontinuity) in the existing circuitry, or to connect a device to the existing circuitry to perform the intended function of the overall circuit design (for a more detailed discussion of bonding applications see U.S. Pat. No. 5,079,070 to Chalco, et al., and U.S. Pat. No. 4,970,365 to Chalco, incorporated herein by reference).
One bonding method is called diffusion bonding, where surfaces to be bonded are pressed together and heated until atomic diffusion takes place. Another bonding method involves soldering, where solder alloys are melted while in close contact with the bonding surfaces.
Microbonding is normally performed with a conventional bonding tip provided by commercial bonders. In these type of bonders, the tip is normally energized by a pulse of ultrasonic vibration that generates friction energy that supplies heat needed for bonding. In some circumstances, external heating is supplied to the tip, when friction heating is not sufficient. In this case, it is desirable to confine the external heat to a very small volume of the bonding end of the tip. This achieved by applying heating pulses (e.g., a laser) for very short duration (10-500 milliseconds). See also U.S. Pat. No. 4,970,365 supra.
In order to achieve a highly reliable bond with an energized tip, the key process parameter to control is the tip temperature. Tip temperature is typically measured using conventional temperature sensors such as thermocouple devices.
A thermocouple is a source of electrical potential (e.g., a battery). It is made of two dissimilar alloys referred here as thermocouple metals A and B. Depending on the materials selected for metals A and B, either A or B can be the positive or negative pole of the battery that the thermocouple represents. Metals A and B come in contact with each other at a point called the junction which is the sensing point of the device. When temperature is applied to the junction, a thermo-electric potential difference develops across the two different metal alloys. This potential difference (or voltage) can then be translated into a temperature reading. This is a highly reliable method of measurement if the mass of the thermocouple junction is at least two orders of magnitude smaller than the mass of the heated mass of the tip.
However, in microbonding the heated mass of the bonding tip is of the same order of magnitude as the mass of the junction made with conventional thermocouple wires. In theory very fine thermocouple wires could be used, but the smallest size produced has a diameter equal to 25 microns, which is still significantly larger than the desired size of 5 microns or less. Therefore, inaccurate temperature measurements are incurred as a result of high thermal dissipation associated with conventional thermocouple wires.
Additionally, 25 micron wires are smaller than a human hair and therefore are very fragile to handle during positioning under the tip. This requires a very tedious alignment procedure under a high-power microscope. This is unacceptable in a production environment where reproducible readings are expected without costly production delays.
Therefore, what is needed is a reliable tip temperature device that can be used in a manufacturing environment. Preferably this device should be based on the thermocouple-wire principle but it must yield reliable, accurate and consistent readings over an extended life of several thousand readings per device, and it should allow easy replacement, by the operator after completion of its useful life.